Bacterial pneumonia is a leading cause of death. Recruitment of neutrophils into the lungs is one of the most important defense mechanisms in the initial host defense against bacterial infection. However, excessive influx of neutrophils can cause extensive lung injury and ARDS, suggesting that neutrophil influx is tightly regulated. Neutrophil trafficking is primarily dependent on chemokine production by myeloid and resident cells in the lung. In the past, most of the attention has been focused on the role of myeloid cells in neutrophil trafficking. Recently, we reported for the first time that resident alveolar epithelial type (AE) II cells produce the neutrophilic chemokine, CXC chemokine ligand (CXCL) 5, in LPS-mediated lung inflammation. In addition, we showed that CXCL5 blockade attenuated LPS-induced neutrophil influx in the lung. In this proposal, we focus on Legionella pneumophila (Lp), as our preliminary data show that the in vivo depletion of CXCL5, but not other neutrophilic chemokines, such as KC and MIP-2, impairs host defense against Lp despite the fact that depletion of either CXCL5, KC or MIP-2 impairs host defense against E. coli infection. We hypothesize that Lp-induced CXCL5 is a critical mediator of neutrophil influx in the lung and CXCL5 production stimulated by Lp infection involves both direct and indirect cascades. The direct cascade involves interaction of Lp with AEII cells and the indirect cascade involves interaction of Lp with myeloid cells leading to the production of inflammatory mediators, which can then stimulate AEII cells. The Specific Aims of this application are: 1) To directly assess the contribution of CXCL5 to neutrophil influx in Lp pneumonia; 2) To delineate the direct pathways responsible for CXCL5 production in murine and human AEII cells (in vitro) after Lp infection; and 3) To delineate the indirect pathways (in vivo) that mediate CXCL5 production and neutrophil influx in the lung during Lp infection. Overall, the proposed studies focus on the novel pathways responsible for CXCL5 production and neutrophil influx in Lp pneumonia. A unique combination of in vivo (mouse, including CXCL5 knockout) and in vitro (murine AEII and dendritic cells, and human AEII cells) systems will be employed to address the Aims. Elucidation of the mechanisms by which Lp induces CXCL5 production and neutrophil influx in pneumonia will lead to a better understanding of disease pathogenesis and ultimately lead to new strategies to the treatment of lung injury and ARDS in bacterial pneumonia. PUBLIC HEALTH RELEVANCE. Bacterial pneumonia is an important lung disease in both adults and children, and affects more than 1 million adults with 30,000 deaths per year in the United States alone. Despite the fact that some advances have been made in the recent past in understanding bacterial pneumonia, we still do not have effective control measures. Neutrophil, a white blood cell, recruitment to the lungs is one of the important protective mechanisms against respiratory bacterial germs; paradoxically excessive accumulation of neutrophils in response to bacteria can significantly contribute to lung damage. A better understanding of the mechanisms underlying neutrophil influx is crucial to designing novel and innovative treatment strategies to minimize excessive lung inflammation. To investigate the mechanisms by which neutrophils are recruited to the lung, we propose to use a model of lung disease (pneumonia) induced by the germ, Legionella pneumophila. L. pneumophila causes severe pneumonia known as Legionnaires disease and is characterized by extensive neutrophil accumulation. We have recently shown the importance of a neutrophil attracting molecule, CXCL5, in the lungs in disease (pneumonia) progression. In this proposal, we will determine the role of CXCL5 in bacterial pneumonia caused by the germ, L. pneumophila. The results from this study will help us understand the role played by CXCL5 in inducing neutrophil accumulation in the lungs in bacterial disease (pneumonia). It is anticipated that these investigations will lead to the development of new and innovative treatment strategies to treat lung diseases via manipulating neutrophil numbers in the lung.